Many wireless communication systems, such as cellular TDMA radiotelephone systems employ some type of mobile unit location apparatus and techniques. Multi-lateration mobile unit location techniques are well known which employ a plurality of base station receivers to triangulate or otherwise determine the location of a mobile unit based on a plurality of different sites.
Other wireless communications systems, such as those employing code division multiple access (CDMA) channelization may use a plurality of CDMA carriers (e.g., codes) over a plurality of different frequencies. A problem can arise with mobile location techniques of CDMA type systems since mobile units typically control their output power as a function of their proximity to a transmitting antenna. For example, as a mobile unit gets closer to a base site antenna, it may lower its output power to allow other mobile units to use the same CDMA carrier. This can provide less interference for other mobile units using the same CDMA carrier. However, as the mobile unit decreases power, the path loss can increase and multi-lateration location can become more difficult since other antennas at other sites may not be able to detect the mobile carrier of the low output power. Accordingly, there is a desire for single site mobile unit location techniques.
Such a single site mobile location system or a multi-site location system may not exist on many cellular base stations. Consequently, the addition of location finding equipment may be required as add-ons to cellular base stations. However, a problem can arise since the addition of single site mobile unit location systems can require additional hardware and/or antennas such as additional CDMA receivers, local oscillators and other front end receiving equipment. This is partly due to the need to provide a mobile unit location apparatus that is compatible with different manufacturers of cellular base stations. However, the duplication of equipment can greatly increase the cost of the overall system.
In addition, for multi-carrier radio telephone systems, such as that defined by IS 95 Proposed EIA/TIA INTERIM STANDARD Wideband Spread Spectrum Digital Cellular System Dual Mode Mobile Station Base Station Compatibility Standard, Apr. 21, 1992 incorporated herein by reference, it is desirable to perform remote unit location determination for many remote units within a single carrier as well cases wherein multiple remote units are assigned to multiple carriers. However, to locate all remote units on traffic channels, duplicate receiver hardware may typically be required to acquire baseband data for multiple carriers. Generally, if wideband receivers are used, such as scan receivers, multiple digital acquisition paths will still be required since remote units to be located may be assigned unique carriers. Thus, locating remote units assigned to unique CDMA carriers requires observation of baseband data within each carrier, which implies the need for digital acquisition paths assigned to each carrier. Further, each of these assigned carriers requires either a wideband multi-carrier receiver, a plurality of multi-carrier receivers, or a plurality of single-carrier receivers.
Moreover, single site location techniques, or multi-site location techniques, may employ time of arrival and direction of arrival algorithms. Yes. It is known in the art to use phased array antennas for determining direction of arrival of incoming signals from remote units. Yes: The use of phased array antennas enables direction of arrival detection by collecting identical copies of the remote unit's transmission at each antenna element. The arrival of these identical copies will have delays at each antenna element that can be evaluated using geometry to determine the angle of arrival. Multipath components of a signal can be descerned up to the design of the phased array and the capabilities of the direction of arrival algorithms.
In addition, access probe signals are also defined in CDMA IS 95C. These access probe signals are typically used by a remote unit to indicate to a base station that the remote unit is available. These access probe signals are typically also generated when a call is sent to begin call set up and use of traffic channels. In the instances where a service, such as an emergency service, wants to know the location of a remote unit as fast as possible, a call may get blocked during channel set up due to congestion or other reasons. This can prevent the location of the remote where only traffic channels are used to determine a remote unit's location. It would therefore be desirable to provide location data for a remote unit prior to traffic channels being used for the communication.
Consequently, a need exists for a remote unit location system and method that can be used in a multicarrier radiotelephone or wireless system while reducing the cost of implementation. It would be desirable if such a system could be used both for single site remote unit location and multi site remote unit location. Such a system and method should also attempt to reduce delays required to obtain location data for selected remote units.